THE SEA SHORES 



185 



alimentary tracts when analyzed corresponds to the detritus on 

 the ocean floor, and the free-floating plants are only inciden- 

 tally present. In the deepest water the organic matter is prob- 

 ably chiefly formed by the free swimming organisms dying and 

 raining downward from above. 



2. Plants growing on the bottom, chiefly eel-grass, upon 

 which browse certain snails, like the periwinkles, a few echi- 

 noderms, and some crustaceans. The Danish naturalists have 

 found that as a basis for the support of the shore living ani- 

 mals these plants are next in importance to detritus. 



3. Free swimming microscopic plants, similar to those of the 

 open ocean. The Danes have found that these are of almost 

 no importance on their coasts; their slight value is indirect, 

 through the medium of the free swimming copepods. But 

 probably elsewhere, especially in arctic and antarctic regions 

 where there is no eel-grass and they are enormously abundant, 

 they become of much significance. 



4. Drift wood, floating or stranded in the water, and wooden 

 structures, such as piles and wharves. These, essentially vege- 

 table detritus, form the food of curious aberrant bivalves 

 called ship-worms or teredos which bore into them and often 

 cause enormous damage. Other bivalves and various crus- 

 taceans, such as the gribble, bore into wood and are often quite 

 destructive, but the teredo is the only creature known actually 

 to live upon it. 



